The growing concern over energy conservation prompted by limited natural resources has produced a need in the art for further improvements in the design of heating systems which increase operating efficiency and thereby reduce energy consumption. In combustive type heating systems, particularly those burning fossil-type fuels such as coal, gas, or oil, products of combustion and toxic gases are generated and must be drawn away and exhausted from the heating system into the open atmosphere, usually by means of a conduit or "flue pipe". As an energy wasting consequence of this type of system, the exhaust gases include a significant quantity of heat produced by the combustion process which is ordinarily lost to the atmosphere and therefore is not effectively employed for, and does not contribute to, the heating of the area which the heating system is intended to warm.
Others in the past have recognized the important advantage of recovering heat from the exhausted combustion gases, and have attempted to devise means for transferring a small portion of such exhausted heat to the area intended to be heated. These prior art systems are typically employed in forced air-type heating systems for buildings, and include a heating chamber provided with a warm air delivery duct and a cool air intake or return duct while a flue pipe for carrying away the gases and products of combustion is usually in communication with the heating chamber. The flue pipe, which normally comprises a metallic, heat conducting material, is disposed within the cool air return duct so that the cool air returning to the heating chamber passes over the warm surface areas of a short stretch of the flue pipe and is, in effect, slightly preheated prior to return to the heating chamber. In this manner, the air within the heating chamber to be heated is slightly increased, consequently, the energy required to elevate the temperature of the preheated return air to the desired temperature is slightly reduced.
Prior art systems of the general type described above are shown in U.S. Pat. Nos. 2,225,181 and 2,439,109. U.S. Pat. No. 2,225,181 discloses a rather large, concentric shell structure interposed between stretches of the flue pipe and in the path of cold air returning to the furnace through a return air duct, which is designed to transfer a portion of the heat flowing through the flue pipe solely by means of heat conduction and relies on a substantial surface area to maximize the heat transfer process; the apparatus shown by this patent is of limited effectiveness in recovering the wasted heat energy, particularly in view of the substantial size and space requirements for the heat recovering structure. U.S. Pat. No. 2,439,109 also shows the use of heat recovery structure interposed between stretches of a flue pipe disposed within the path of cool air flow in the return duct of a furnace, and includes radiating members presenting a relatively large surface area to the cool return air flowing thereacross in order to maximize the transfer of heat from the gases within the flue by conduction processes to the cooler return air; the heat recovery structure disclosed by this patent requires the use of a baffle means for diverting the combustion gases in the main stream of the flue pipe away from the latter and into the radiating structures. The apparatus disclosed by the latter mentioned patent is also undesirable from the standpoint that the heat exchanging structure is rather large in comparison to the quantity of heat which it recovers, and necessitates the provision of a cool air return duct having a relatively large cross-sectional area to accommodate the same.